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This answer contains spoilers… (view spoiler)[Some spoilers for On the Shoulders of Titans below, as well as some discussion of content for future books.

That concept does exist in this setting, and it's referred to as a mana battery. Mana batteries definitely do exist, but they're not quite as useful for novice enchanters as they might sound.

Normally, if you're transferring mana from one item to another, you're briefly passing it through your body in the process. Mana is "impure" by default, and putting impure mana into your body can cause harm. Purification is an issue if you intend to run the mana through your body at all.

There are a few ways to work around this.

High level Enchanters can learn to purify mana. In book 1, Vellum says, "Even some Citrines cannot do this through their attunement alone." So, it's definitely a difficult skill to learn.

You could also get an attunement that specializes in purification, like the Arbiter attunement does.

You could also figure out a way to transfer mana between two objects without using your body as an intermediary. This is absolutely possible, and higher level Enchanters do it, but it's generally inefficient because mana disperses rapidly when it's not contained in something like a rune or a crystal.

The other major issue is transfer speed. In order to successfully charge a rune, you need to be able to fill it up completely in a limited time frame, which varies from rune-to-rune. Taking mana out of one rune to put it into another is slower than moving your own mana into a rune or moving it from a crystal, since you're effectively working against the rune's "resistance" to pull the mana out of it. This means that pulling all the mana out of it quickly enough to successfully charge another rune is challenging, and requires a higher degree of mana manipulation skill than most low-level Enchanters have.

In On the Shoulders of Titans, Corin's second attunement provides him with automatic mana purification as well as an improvement to his mana transfer speed. With these advantages, making things like low-level batteries may be possible. He would still have difficulty with, say, grabbing mana from several different low-level batteries to charge a high level rune - that's something he'd have to work toward in the longer-term.

Low level batteries are definitely on his list of things to make, he just has had more immediate needs to prioritize, since low level batteries don't have much immediate combat utility - they'd be more of a long-term investment. (hide spoiler)]

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